Osteolepiforms and the ancestry of tetrapods

Abstract

Fossil discoveries1,2,3,4,5,6,7 and improved phylogenies3,4,5,7 have greatly improved our understanding of the origin of tetrapods, making it possible to reconstruct sequences of character change leading to tetrapod morphologies5,7 and to tentatively identify the genetic basis for some of these changes8,9. However, progress has centred on the upper part of the Tetrapodomorpha5 which is occupied by Devonian tetrapods such as Acanthostega1,2,5 and Ichthyostega1. Few advances have been made in improving our understanding of the lower, ‘fish’ part of the group, beyond establishing Elpistostegalia, Osteolepiformes and Rhizodontida as progressively more primitive constituents10,11,12,13. It has not been convincingly confirmed or disproved that the Osteolepiformes, a diverse but structurally uniform group that is central to the debate about tetrapod origins14,15,16,17, is monophyletic relative to tetrapods (that is, a single side branch on the tetrapod lineage). The earliest steps of the fish–tetrapod transition have thus remained poorly resolved. Here we present the first detailed analysis of the lower part of the Tetrapodomorpha, based on 99 characters scored for 29 taxa. We show that both the Osteolepiformes as a whole and their constituent group Osteolepididae are paraphyletic to tetrapods (that is, each comprises a section of the tetrapod lineage with several side branches), and that their ‘uniting characters’ are attributes of the tetrapodomorph stem lineage. The supposedly discredited idea of osteolepiforms as tetrapod ancestors14,15,16,17 is, in effect, supported by our analysis. Tetrapod-like character complexes evolved three times in parallel within the Tetrapodomorpha.

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Figure 1: Previous osteolepiform phylogenies.
Figure 2: New phylogeny of the tetrapodomorph stem group (consensus of 9 trees, based on 99 characters scored for 29 taxa).
Figure 3: Ancestral characters for clade Elpistostegalia+Tetrapoda.
Figure 4: Parallel evolution in the tetrapod stem group, inferred from the phylogeny in Fig. 2.

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Acknowledgements

We thank the Australian Museum for their award of a Visiting Fellowship to P.E.A., E. Mark-Kurik for access to material of Thursius estonicus, J. Jeffery for information about rhizodonts and Academic Press for Figs 3a–c, 4a.

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Correspondence to Per E. Ahlberg.

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Ahlberg, P., Johanson, Z. Osteolepiforms and the ancestry of tetrapods. Nature 395, 792–794 (1998). https://doi.org/10.1038/27421

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